In hydrocarbon fuelled fuel cell power systems a reformer is used to reform a hydrocarbon fuel, such as natural gas, methanol, etc, into a hydrogen rich reformate gas which is supplied to the anode chambers of the fuel cell stack. The fuel cell stack may comprise a plurality of phosphoric acid fuel cells or a plurality of solid membrane electrode assembly fuel cells, for example solid polymer fuel cells.
The characteristics of a fuel cell make it intrinsically more efficient at low power than at high power, making the fuel cell well suited, in principle, to low load factor applications. Low load factor applications often require rapid load following response characteristics. Typical low load factor applications include the electric load demand of a residential dwelling, a motor vehicle, etc. Any power system designed to service such applications would spend long periods of time at zero load, but yet be required to start up virtually instantaneously on demand. In principle the low temperature, robust solid polymer fuel cell can respond instantaneously to changes in load demand, making it well suited to such applications.
However, to sustain an increased load demand in practice requires a similar instantaneous response from the reformer. Currently used reformers, for example steam reformers or autothermal reformers, have relatively slow response times and relatively long start up times. A recent reformer concept, which is described in European Patents 0217532 and 0262947, has a faster response time and a shorter start up time than the currently used reformers. Unfortunately the response and start up times of this reformer are still slower than the response time of a fuel cell, such as the solid polymer fuel cell, and are also slower than the response time required in residential, automotive and many other applications.
The electric current drawn from the fuel cell stack will respond instantaneously to a load demand, but as mentioned above the hydrogen supply from the reformer takes a little longer to respond. The transport delay as the hydrogen flows along the pipes is also likely to be significant because of the low flow rates. The reformate in the anode chambers of the fuel cell stack provides some spare capacity, but this is not sufficient to supply the large load changes the fuel cell stack is designed to meet. Thus the hydrogen partial pressure in the anode chambers of the fuel cell stack will fall rapidly, during large power demands, with a consequential fall in output voltage.